1. Field of the Invention
The present invention generally relates to a defect inspection system and is directed more particularly to a defect inspection system which inspects the defects on patterns or the like of an object to be inspected.
2. Description of the Prior Art
It is a general practice to use photomasks which have a predetermined pattern upon manufacturing semi-conductors, printed circuit boards or the like. In such case, when there is a defect on the pattern of the photomask, it will cause the production of reject products so that the photomask pattern defect inspection is an important process in the art.
An example of a defect inspection system for patterns as proposed under the prior art shall be explained in reference with FIGS. 1, 2 and 3 hereunder.
FIGS. 1 and 2 are respectively plan views showing magnified photomask portions by a microscope on which 1 and 1' are respectively photomasks made of transparent material such as glass or the like, 2 and 2' are respectively patterns formed by evaporated metal or the like, in general on photomasks 1 and 1', 3 is the transparent section of the transparent base plate of photomasks 1 and 1', and 4 is the nontransparent or opaque portion of the photomasks 1 and 1' by the evaporated material thereon.
On FIG. 2, A and B are the portions at which the evaporated material unnecessarily remains, while C and D are the portions where the necessary evaporated material is lacking. Accordingly, photomask 1' which has a pattern such as 2' as shown on FIG. 2, is a defective product. On the other hand, the photomask 1 as shown on FIG. 1 is a complete and normal product.
In order to inspect photomasks such as 1 or 1' as shown on FIGS. 1 and 2 under the prior art, for instance as shown on FIG. 3, a standard mask 6 which has a complete pattern (for instance such as pattern 2 as shown on FIG. 1) is placed at a predetermined position on a transparent base 5 as an example, while a mask 7 to be inspected (such as a defective pattern 2' as shown on FIG. 2) is placed at another predetermined location on base 5, whereas both are observed by a binocular microscope 8. On FIG. 3, 9 and 10 are object lenses for both of the standard mark 6 and inspected mask 7, 11 and 12 are mirrors for both masks 6 and 7, 13 and 14 are half mirrors for both masks 6 and 7, 15 is a common eye piece lens, 16 is a light source for instance which irradiates red light on the standard mask 6, and 17 is the light source for a red complimentary colour such as a green light for instance, which irradiates on the mask 7 to be inspected. The lights emitted from light sources 16 and 17 pass through the base 5 of the binocular microscope 8, masks 6 and 7, lenses 9 and 10, mirrors 11 and 12, half mirrors 13 and 14, and further lens 15 and form a focussed image at an observing eye 18, which makes the inspection of a mask possible.
In the case that the inspected mask 7 is a defective product as shown on FIG. 2, at portions A and B, the green light from light source 17 is shielded by such portions A and B, while only the red light from light source 16 arrives at the observing eye 18, and hence such portions A and B appear in red colour. On the other hand, for portions C and D, the red light from light source 16 is shielded but the green light from light source 17 arrives at the observing eye 18 and accordingly, such portions C and D appear in green colour. As for the other portions such as the transparent portion 3, both of the red and green lights from light sources 16 and 17 arrive at the observing eye 18 simultaneously and hence the transparent portion 3 generally appear as white and further, as for the opaque section 4 the red and green lights are both shielded and do not pass and hence the opaque section 4 appears as black generally. In other words, when the entire mask appears in white or black, the inspected mask 7 has no defect and whenever there is a slight shade or red or green visible, the inspected mask 7 shall be a defective product.
The above mentioned defect inspection system for patterns under the prior art contains the fault that the inspection can not be automatically conducted because it is conducted at piece by piece with the human eyes. In the case that this inspection was to be automated, a very high costing colour television camera becomes necessary.